Fasteners are used in a wide variety of applications, such as motors, railroad tracks, flange assemblies, petrochemical lines, foundations, mills, drag lines, power turbines and studs on cranes and tractors. In many applications, achieving the proper fastener tightness (tension) and maintaining this tightness once the system is placed in service is problematic. As is known in the use of such fasteners, as force is applied to a portion of the fastener, e.g. a head of a bolt or the like, a load is applied to the fastener. As the fastener is tightened, this load increases to a maximum break point i.e. where the fastener breaks or its integrity is otherwise compromised. For various applications, optimal loads are known and/or are obtainable, but currently available methods and apparatus do not adequately enable reliable and repeatable determinations thereof.
Fasteners typically experience a loss of tension when placed in service due to, for example, a variety of in-service occurrences including: relaxation (thread embedment), vibration loosening, compressive deformation in the joint or flange, temperature expansion or contraction, etc. The loss of tension that results from these occurrences can cause premature wear in the assembly, leakage (in applications where the fastener is used for sealing), or catastrophic joint failure due to excessively high loads on other members of the assembly. In certain applications, knowledge of a fastener load (tightness), initially and over time, is desirable for avoiding the potentially dangerous consequences of a loosened fastener, such as slippage, wear, leakage and/or possible failure. In other applications, for example when working with a plurality of bolts around a flange, it is important to evenly tighten the group of bolts. By uniformly tightening a group of bolts or studs to an appropriate load, and maintaining this load over time, potential failures are less likely to be experienced.
An apparatus and method is therefore needed to accurately tighten a fastener and to determine the existing clamp load status. Because current fasteners do not reliably indicate the status of the tension in the fastener, users must often use cumbersome methods to check the tightness of each bolt, or simply re-tighten all of the fasteners regardless of whether such re-tightening is needed. The retorquing (i.e. tightening) of a fastener, however, induces wear and strain in the fastener system from the friction, variations in nut condition, torque values, and the like. Furthermore, the applied torque typically does not expose the bolt to stresses above the elastic range of the fastener, otherwise failure of the fastener may result.
Various load indicating fasteners which analyze and/or utilize the elongation of the tightened fastener to determine the existing clamp load status are known, and such fasteners differ greatly in structure as well as in the methods and apparatuses with which they are used. For example, the analysis of the strain (elongation) existing in the fastener may be conducted through the use of mechanical, electrical, optimechanical, ultrasonic methods and the like. See, for example, U.S. Pat. No. 4,676,109 issued Jun. 30, 1987 to Wallace, U.S. Pat. No. 5,388,463 issued Feb. 14, 1995 to Scott and U.S. Pat. No. 2,600,029 issued Jun. 10, 1952 to Stone each disclosing fasteners including various electronic measurement devices; U.S. Pat. No. 4,899,591 issued Feb. 13, 1990 to Kibblewhite disclosing an ultrasonic load indicating member; and, U.S. Pat. No. 4,823,606 issued Apr. 25, 1989 to Milecki disclosing a diaphragm transducer for sensing load.
Typically, these methods are not usable by ordinary workers. For example, electronic or ultrasonic methods for determining existing clamp load require experienced operators, expensive equipment, clean surfaces and records of pre-installation test values for each bolt or stud. Experienced operators must preform numerous calculations to obtain the clamp load, while compensating for deformations in the head of the fastener. Moreover, devices which require complicated electronics tend to add to the expense, maintenance and unreliability of the system. In addition, such systems may be adversely affected by shock and other extreme conditions.
Some of the prior art devices include reference surfaces or points from which the relative displacements must be measured and analyzed. See, for example, U.S. Pat. No. 4,428,240 issued Jan. 31, 1984 to Schoeps and U.S. Pat. No. 3,561,260 issued Feb. 9, 1971 to Reynolds. These systems generally require skilled labor to use complicated and sophisticated measurement techniques. Moreover, instant visual inspection of the load in a fastener is typically not possible. Additionally, the reference surfaces generally are exposed to the outside environment which often leads to outside forces affecting the system.
Some other of the prior art devices include color indicators which denote the load changes within the fastener by changes in the color of the indicator. See, for example, U.S. Pat. No. 3,987,668 issued Oct. 26, 1976 to Popenoe, U.S. Pat. No. 3,823,639 issued Jul. 16, 1974 to Liber and U.S. Pat. No. 3,964,299 issued Jun. 22, 1976 to Johnson. These indicators require interpretation of the color designation and only indicate when a load exists. Determination of intermediate load levels, or partial loosening of a fastener, is not possible. Furthermore, because most fasteners are in-service in an outdoor environment, variations in sunlight may restrict an inspector's ability to determine the specific color of the indicator.
In general, the present inventor has found that mechanical mechanisms generally provide the most reliable devices for sensing the load in a fastener. For example, UK Patent Number GB 2-265-954-B published May 31, 1995 to Ceney discloses a load indicating fastener with a U-shaped load sensing unit. A first end of the load sensing unit is anchored to the fastener, while the apex of the member sits against an abutment within the fastener. A second end of the load sensing unit acts as an indicator by registering the load on a scale. While this device has some utility in certain applications, over time, the device can become inaccurate and lose calibration. This loss of calibration has been found to be inherent in the design of the U-shaped sensing element, because as the element is continually strained, its calibration characteristics are inherently affected. Thus, after a number of cycles, the U-shaped element may need to be replaced. In addition, temperature may effect the bending characteristics of this U-shaped element. Also, when attempting to match coefficients of linear expansion between the bolt and the U-shaped element, one is limited due to the fact that only a limited number of materials are sufficiently resilient to be used for this design. Deformation of the sensitive U-shaped element also occurs with relatively small amounts of shock. The bottom of the U is in constant contact with the abutment and any shock will be transferred at the bottom of the U and tend to deform the U-shaped element causing it to lose calibration.
Still other prior art designs use external indicators. For example, the Ceney patent, UK Patent Number GB-2-179-459-A, discloses an externally mounted mechanism for indicating the tightness of a fastener. This system includes a pin in the bore of the fastener that extends out of the end of the fastener, upon extension of the bolt, the pin applies pressure to fulcrumed levers positioned perpendicular to the axis of the bolt. The levers, which are acted upon by a compression spring, are visible through a window in the cover. Due to the design of this system, and the complex arrangement of the levers, and position of the indicator window,the indicator components must be positioned on the outside of the bolt, which in some applications is not possible due to space restrictions. In cases where it is possible to use such a configuration, the elements of the instrument may be susceptible to outside forces and damage. Upon damage, no convenient method exists to verify whether or not the unit is still calibrated.
Because the prior art apparatuses and methods are time consuming, skilled labor intensive, extend outside the fastener, subject to environmental conditions, unreliable, cannot operate at high temperatures and require expensive measuring equipment, a device is needed that, upon tightening of the fastener, indicates the amount of strain from the elongation of the fastener. Thus, a need exists for a tension measuring or indicating device that is simple, easy to manufacture and inexpensive. Moreover, a need exists for a rigid, substantially internal and more durable system which will not experience the aforementioned bending and decalibration problems and which allows accuracy (calibration) to be easily verified in the field.